Seem to either totally wreck, or get absolutely trashed on...

There’s no inbetween. I realize it’s an alpha, but will there be any sort of incentive to balance teams? Maybe have a reduced score or something if the game can recognize unbalanced teams that punishes the other team for not switching sides. I don’t know, but I was in a match where my whole team only died like once and everyone had 10+ kills. Now I was just in a match where the same thing was happening to me.

I think the prevalence of stomps might also have to do with spawn times and low populations on both teams. In Chiv we often saw that players who trickled in from spawn alone rather than grouping up often died faster than if they had approached the enemy with even numbers. With only 6 players on each side, a single player trickling in to the fight is worth 1/6 of your team whereas a solo player in Chiv might be worth 1/16 of your team.
High spawn times exacerbate this problem because they require players to wait a longer time if they want to go together toward the fight. Let’s say that you’re playing “push the cart into the enemy base” on Lighthouse and both teams meet 6v6 in the center. If the battle lasts 30 seconds and a player on orange team dies every 5 seconds as they are gradually picked off, this places a high burden on the first player who died to not only wait the 15-20 seconds to respawn but also another half a minute to group up with all 5 of their teammates. Even if the purple team lost 3 of their players in the first fight, Lighthouse is such a small map that they will probably be able to push the cart almost all the way to the end which forces the players who have respawned on the orange team to charge toward the cart in the hope of stalling it. As individuals rush into a 1vX they will likely die, accomplish nothing, and effectively doom their team. But, with such a fast-moving cart on such a small map, it doesn’t feel like they have a choice to be patient.
All of these factors combined result in more uneven fights after the first and give the implication that the first fight will likely decide the round. This makes teamplay even more important in Mirage and, while I think that it’s good to encourage coordination, the spawn times contribute to the feeling of being stomped because the spawn times effectively force more 1vX fights.